1. Field of the Invention
The present invention relates to a suspension control system for controlling the attitude of a vehicle.
2. Prior Art
There have conventionally been proposed various types of active suspension systems which are designed to suppress changes in the attitude of a vehicle, such as rolling, which occurs during cornering, and squat or dive phenomenon, which respectively occur when a vehicle is rapidly starting or being braked, so as to improve the driving reliability and ride of the vehicle and to control the height of the vehicle. Explanation will be given for one example of such a suspension control system in reference FIG. 1.
Each suspension unit 1 disposed between a vehicle body and respective wheels comprises a cylinder 2 which extends and contracts in response to the supply thereto and discharge therefrom of hydraulic fluid to adjust the relative displacement between the vehicle body and the wheel, an accumulator 3 connected to the cylinder 2 and designed to act as a spring element, and a throttle valve 4 disposed in the line connecting the cylinder 2 and the accumulator 3 and designed to generate a damping force. It should be noted that the figure only shows two suspension units, either for the front wheels or for the rear wheels. Since the two suspension units are controlled in the same manner, the following explanation will be given with respect to one of them.
A supply and discharge means 8 is disposed between lines 6 and 7 for each suspension unit 1, which lines are respectively connected to the cylinder 2 of the suspension unit 1 and to a hydraulic fluid supply source mechanism 5 for supplying hydraulic fluid to the cylinder 2. The supply and discharge means is provided with an electromagnetic proportional control valve 9 of a three ports and three positions type. When the solenoid of the control valve 9 is energized, the control valve 9 is selectively switched from a neutral position a to a hydraulic fluid supplying position b and a hydraulic discharging position c to respectively supply and discharge the hydraulic fluid to and from the suspension unit 1. Reference numeral 10 designates a pilot check valve which is provided in order to prevent leakage of the hydraulic fluid from the cylinder 2 to a reservoir tank 11, since hydraulic fluid may otherwise leak through the control valve 9 when the control valve is in the neutral position a. The pilot check valve 10 is controlled by means of an electromagnetic switching valve 12 of a three ports and two position type. When the switching valve 12 applies a pilot pressure to the pilot check valve 10, the pilot check valve 10 is opened to connect the cylinder 2 with the proportional control valve 9. When the switching valve 12 does not apply any pilot pressure to the pilot check valve 10, the pilot check valve 10 acts as a normal check valve to allow the flow of the hydraulic fluid from the control valve 9 to the cylinder 2 and to prevent the flow of hydraulic fluid in the reverse direction.
Next, explanation of the hydraulic fluid supply source mechanism 5 will be given. The hydraulic fluid supply source mechanism 5 comprises a pump 13, a check valve 14 which prevents the hydraulic fluid discharged from the pump 13 from flowing back to the pump 13, an accumulator 15 which accumulates therein the hydraulic fluid discharged by the pump 13, a pressure detecting means (not shown) for detecting the pressure in the accumulator 15, an unload line 16 connecting the discharge port of the pump 13 and the reservoir tank 11, and an unload valve 17 disposed in the unload line 16. The unload valve 17 is an electromagnetic switching valve. With the constitution mentioned above, the hydraulic fluid discharged from the pump 13 is accumulated in the accumulator 15 and, when the pressure detecting means detects a predetermined pressure in the accumulator 15, the unload valve 17 is switched over to return the hydraulic fluid discharged from the pump 13 to the reservoir tank 11, thereby retaining the pressure in the accumulator at a predetermined level.
It should be noted that the valves 9, 12 and 17 are controlled by means of a control unit (not shown), which receives signals from a speed sensor, steering angle sensor, acceleration sensor, vehicle height sensor and so on (not shown), and outputs directing signals to the valves for retaining the inclination and height of the vehicle at the optimum condition in response to the signals from the sensors.
However, the suspension control system explained above suffers from the following problems.
When the system is in the on-load condition where the hydraulic fluid discharged from the pump 13 is supplied to the suspension unit 1, the engine bears a heavy load, while the engine bears little load when the system is in the unload condition where the hydraulic fluid discharged from the pump 13 is returned to the reservoir tank 11 through the unload valve 17. When the system is switched between the on-load and unload conditions, therefore, the engine suffers from large changes in torque, thereby causing a deterioration in driving reliability and an erratic and uncomfortable ride.
In addition, when the system is switched over between the on-load and unload conditions, shock noise and vibration are generated by virtue of surge pressure which is generated by the sudden opening or closing of the unload valve 17.